It's not Rocket Surgery: Choosing an Indoor Mini Wood stove for a Tiny House

by Geoffrey Chia, originally published by Resilience.org
| Feb 8, 2016

As of 25 January 2016 at the time of commencement of writing this article, the cold spell affecting East Asia killed more than 65 people in the subtropical island of Taiwan. The temperature in Beijing dropped as low as minus 40 degrees C, however Northern China is used to dealing with heavy snow dumps and subzero temperatures in winter and hence coped better compared with Taiwan. The last time snow was seen in Taipei, the capital of Taiwan, was more than a century ago. The fact that Taiwan is a relatively small subtropical island, which we would expect to be dominated by the moderating effect of the South China Sea, makes this rare event seemingly even more bizarre. That is, until you understand the physics of how coastal and island provinces on the leeward (downwind) side of continents can often be dominated by continental rather than maritime weather, which I mentioned in my previous article "Location, location, location", where I compared Vancouver Island with Nova Scotia. http://www.doomsteaddiner.net/blog/2016/01/14/location-location-location/

It is no coincidence that a record breaking monumental snowstorm also affected the East coast of the USA, the leeward side of the North American continent, at the same time as the cold weather affected East Asia. In general, at those latitudes; the windward (Western) sides of the continents, which are more dominated by maritime influences, are less prone to extremes of heat and cold than the Eastern sides.

Climate scientists tell us the peculiar combination nowadays during Northern winters of high temperatures in the Arctic regions and low temperatures extending to low latitudes, is due to the disruption of the circumpolar jet stream as a consequence of climate change.

One issue I did not mention in my "Location" article was the beneficial warming effect of the Gulf Stream on North Western Europe. The demise of the Gulf Stream appears to be a foregone conclusion as climate change progresses, following which North Western Europe can expect even worse cold spells in winter, even as the rest of the world heats up.

My "Location" article focused on avoiding heatwaves and managing fresh water supply. It did not discuss coping with cold weather (which well prepared humans can survive better than heatwaves). In choosing our future location, it is far better to settle where there will be low risk of heatwaves (despite the occasional cold snaps), rather than to settle where there will be low risk of cold snaps (but have high risk of terrible summer heatwaves). Most locations in our future world will face the latter situation in due course.

This article will focus on one item which can keep you comfortable in cold weather, especially when faced with the inevitable fossil fuel shortages in the near future: the indoor wood heater/stove, with special attention to the secondary combustion biomass (SCBM) heater/stove. It is not a comprehensive article but will concentrate on models which can be used in a caravan or tiny house, my particular area of interest.

The other important role of this biomass heater/stove will of course be for cooking. It will become an essential appliance when we face future shortages of kerosene and LPG.

Before we proceed, it is vital to mention the first three principles in dealing with cold weather: insulation, insulation, insulation (just as the first three principles in managing electricity use are efficiency, efficiency, efficiency). The well insulated tiny house, warmed by two people and a dog, may not require any additional heating because of the small interior volume of air.

Good insulation, small interior volume and thermal mass (heated by daytime passive solar influx) alone may enable you to go without active heating for most of the year, depending on where you live.

On the other hand, for adequate ventilation and to avoid condensation, it is necessary to allow some fresh (cold) air in and stale (warm) air out of the tiny house. In the case of the hermetically sealed modern Scandinavian dwelling, this is achieved (with minimal temperature drop) by a heat recovery ventilation system. For a less airtight dwelling in a less cold climate, sufficient ventilation may occur through "natural" leaks in your house (eg air coming in via the gap under the front door and out via a poorly sealed upper window), which will of course cause a drop in internal temperature. If you are installing a wood stove in your tiny house for future cooking anyway, this stove can double up as your heater and there will be no need for an electrically powered heat recovery ventilator. In this article I use the terms biomass and wood interchangeably, because 99.9% of the time, most of us use wood for our (non fossil fuel) stoves.

My criteria for the ideal biomass stove / heater for a tiny house are as follows:

Criteria 2 and 3 can be summed up in one word: efficiency. The least efficient, most hazardous, most polluting and most wasteful heat source is an open fire. Next worst is the open brick fireplace. The standard cast-iron combustion box with flue is a good deal better but still woefully inefficient. Furthermore even a "small" cast-iron stove can easily weigh 150kg, which is quite unsuitable for a tiny house on wheels.

Even though the technology has been around for decades, we have failed to widely embrace the secondary combustion biomass (SCBM) stove, of which the "rocket stove" is the commonest design. Standard fires only burn the primary solid material of the biomass, releasing secondary combustible materials such as soot, hydrocarbon gases and carbon monoxide into the atmosphere, which can cause acute irritation of the airways and eyes, even poisoning or asphyxiation. If inhaled chronically over decades it can lead to emphysema and even lung cancer. These remain terrible problems in Third World countries. A stove designed to burn both primary biomass as well as the secondary emissions is far more efficient, causes far less pollution and is thus far healthier than traditional wood stoves. It needs far less fuel to do the same work (eg just a quarter of the wood normally used for a conventional stove), hence requires less back breaking physical effort chopping and carrying wood, hence also protects against deforestation. Additionally such a stove can reach much hotter temperatures. There have been great initiatives to introduce SCBM stoves (such as the InStove) for use in the developing world, if only for health reasons.

If you google "rocket stove", the vast majority you find will be solely for outdoor use and will lack exhaust flues. Even those with exhaust flues may not be certified for indoor use (the commercial forces of our fossil fuel economy have long suppressed this market). Hence most "permies" these days are pioneering this option without official sanction.

Development of the "rocket mass heater" actually preceded invention of the rocket stove. This mass heater is a different beast, in that the hot metal components are enveloped in big heavy slabs of cob (or similar earthen or concreted thermal mass) to retain the heat for slow and steady release even after the fire goes out. The cob can be shaped in the form a comfortable warm bench. If used indoors, this mass heater is only suitable for a large, fixed, ground based dwelling, not a tiny home on wheels. Furthermore many rocket mass heaters may not incorporate a stove in their design.

With regard to SCBM stoves with exhaust flues/chimneys (which may or may not have been officially approved for indoor use) my websearches revealed the following types:

Click for larger version.

The above are mere impressions obtained from web searches, not based on any practical experiences of mine. Practical reviews from the manufacturers and customers can be found from amazon.com or youtube. This article is intended to spark (pun intended) interest in this topic so that the reader can do their own research and make their own decisions. One issue I have not yet looked into is the maintenance required for each stove type. Every setup will require periodic cleaning of the chimney.

Based on size and availability, the only two SCBM contenders for use in a tiny house on wheels are the Silverfire Hunter and the Kimberley stoves. Although the Kimberley is far superior in every way it is also hugely more expensive, however you generally get what you pay for. It is false economy to buy a cheap stove if your house ends up burning down.

The Silverfire Hunter is described as a "toplift updraft" or TLUD gasifier. It generates bare flames and smoke out of the central cavity at startup (secondary combustion occurs later in a ring around this). This issue may be a dealbreaker for the indoor user. During cooking, the base of the pot/pan is in direct contact with the primary flame, causing soot deposition. An optional cast iron disc cover is available to minimise this, although it will also reduce heat transfer. One reviewer wrote that it needs frequent ash removal which requires it be disconnected from the flue, taken outdoors and turned upside down.

For those still keen on mini wood stoves of standard design, the following review websites are helpful:

As mentioned previously a small standard cast iron stove can easily weigh 150kg, however the tiniest models may weigh less than 25kg. The "Vista" stove, by Pacific Energy on Vancouver Island BC, is approved for mobile home use, but weighing in at well over 100kg is not a serious contender from my point of view.

For my purposes, two particular models seem especially suitable (mainly because I greatly value the ability to source external air intake):

Jotul 602: apparently more than a million of these have been made, hence one would expect all the bugs have been ironed out by now. H25.25” x W12.6” x D21.25” , costs around $900. Weighs 73kg, able to configure for external air intake. Apparently 75% efficient (?capable of some secondary combustion)

Size: 302mm wide, 272mm deep and 465mm high, the low emission version costs £525 pounds sterling. Weighs 60kg, able to configure for external air intake. It is designed to enable some secondary combustion, although not as efficient as the purpose designed SCBM stoves. Option to incorporate a water boiler which can add to thermal mass.

CONCLUSION:

As in all things, your choice will depend on how you weigh up the various advantages versus disadvantages of each stove, as well as your individual circumstances. Many amateurs have cobbled together home-made rocket stoves for outdoor use, however few sane people will hazard home-made stoves for indoor use.

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